This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Abbreviations that may be found in the specification and/or the drawing figures are defined at the end of this document, prior to the claims.
3GPP LTE Rel-12 will be part of an LTE-Advanced system. The instant disclosure focuses on PUCCH resource allocation for HARQ-ACK related to dynamic UL-DL reconfiguration for TD-LTE (or TDD mode of LTE). As is known, HARQ-ACK is a technique for sending error information indicating received information was not correctly received. The description herein falls within the scope of Rel-12 Work Item eIMTA—“Further Enhancements to LTE TDD for DL-UL Interference Management and Traffic Adaptation”. The Work Item Description for eIMTA (RP-121772 by CATT) was approved at RAN#58 meeting, Barcelona, Spain, Dec. 4-7, 2012.
The goal of the eIMTA WI is to enable more flexible TDD UL-DL reconfiguration for traffic adaptation in, e.g., small cells. The starting point following the Rel-12 assumptions is that the eNodeB may vary UL-DL configuration relatively often (e.g., for those UEs configured to a flexible UL/DL mode) compared to the existing situation where UL-DL configuration is in practice very stationary. For instance, TD-LTE Rel-11 supports UL-DL reconfiguration with periodicity of 640 ms or longer. The basic assumptions for eIMTA functionality, based on RAN WG1 progress, are listed below:                There is a predefined cell-specific UL-DL configuration broadcasted in the cell using SIB-1. The legacy UEs (Rel-8 to Rel-11) in the cell follow this configuration all the time.        No new TDD UL-DL configurations are introduced: Flexible TDD reconfiguration can only happen among existing (e.g., seven) configurations. These configurations are described in more detail below.        TDD reconfiguration can occur with (at most) radio frame (=10 ms) periodicity for those UEs configured to the new flexible configurations.        In each UL-DL configuration, there are fixed subframes where the link direction is always predetermined. These fixed subframes are denoted as D (Downlink), S (Special) and U (Uplink)        Additionally, there are as well flexible subframes (denoted as F). Flexible subframes can be used as D or U. From the measurement point of view of legacy UEs, it may be possible to use a special subframe (e.g., subframe #6 of the radio frame) only as S or D (but not U).        The number of flexible subframes depends on the scenario (such as SIB-1 configuration and DL reference configuration defining HARQ/scheduling timing for PDSCH).        
Regarding the existing TDD UL-DL configurations, FIG. 1 illustrates UL/DL configurations for TD-LTE (Rel-8/9/10/11). Physical PUSCH resources are located between two PUCCH segments shown in FIG. 2. Mapping of logical PUCCH resource blocks, denoted as m, into physical PUCCH resource blocks is also shown in FIG. 2. Taking into account the logical split between different PUCCH Formats, it is noted that PUCCH Format 2/2a/2b carrying periodic CSI reports is located at the outermost resource blocks (either near RB0 or near NRBUL−1) of the system bandwidth. ACK/NACKs for persistently scheduled PDSCH and SRIs are located on the PUCCH resource blocks next to periodic CSI while the ACK/NACK resources reserved to dynamically scheduled PDSCH are located at the innermost resource blocks reserved for PUCCH. It can be noted that the size of dynamic PUCCH defines the continuous spectrum available for PUSCH. That is, the area between the areas for PUCCH may be used for PUSCH.
Turning to FIG. 3, this figure is an example of a radio frame with 10 subframes showing Downlink (D), Uplink (U) and Special (S) subframes according to exemplary SIB-1 configured UL-DL configuration #0, as well as flexible subframes available for Rel-12 UEs configured to flexible UL/DL mode. TDD configuration 0 (zero) is used in FIG. 3, but the same principle applies to other configurations as well. In addition to the SIB-1 configured UL-DL configuration, which defines whether a given subframe in the radio frame is downlink, special, or uplink subframe, in the case of flexible TDD UL-DL configurations, e.g., some of the uplink subframes can be changed from what is shown in the row labeled “SIB-1” into downlink subframes (as can a special subframe be changed into a downlink subframe).
In TDD eIMTA systems, HARQ/scheduling timing related issues are still under active discussions although basic principles have been agreed upon. The remaining open questions in RAN WG1 relate, e.g., to exact HARQ timeline, HARQ process continuity, HARQ-ACK resource allocation, and the like.
In this disclosure, the focus is placed on the HARQ-ACK resource allocation on PUCCH. In the current specifications (up to Rel-11), the PUCCH HARQ-ACK resources are implicitly determined based on the lowest (i.e., first) CCE of the corresponding PDCCH and semi-statically configured PUCCH Format 1/1a/1b starting position. Furthermore, in TDD mode, there is a separate timing offset defined as a function of SIB-1 configuration and the subframe index. From HARQ/timing point of view, the eIMTA feature will create two parallel sets of UEs sharing the same PUCCH Format 1/1a/1b resources:                Legacy UEs following HARQ/scheduling timing according to SIB-1 UL-DL configuration; and        eIMTA UEs following HARQ/scheduling timing according to DL reference configuration.        
This will create PUCCH resource collision problems, which should be avoided by proper system design. Otherwise, additional complexity is involved in the scheduler entity allocating shared PDCCH (and/or EPDCCH) resources between legacy UEs and eIMTA UEs.